Anewly discovered element, z, has two naturally occurring isotopes. 90.3 percent of the sample is an isotope with a mass of 267.8 u, and 9.7 percent of the sample is an isotope with a mass of 270.9 u. what is the weighted average atomic mass for this element?

Ok so, remember that the average atomic mass is what is seen on the periodic table. It is the average mass of all of the isotopes with their frequency taken into account. What you need to do is add the products of the masses and frequencies Just like this:

0.903*267.8 + 0.097*270.9
When you add it the result is what you are looking for

It is 4.92 e-4 sorry if i am wrong